1. Field of the Invention
The present invention relates to a process for producing a spherical catalyst carrier, such as alumina spherical carrier, silica-alumina spherical carrier, zirconia-alumina spherical carrier, titania-alumina spherical carrier, boria-alumina spherical carrier, and boria-silica-alumina spherical carrier. This process permits easy production of a spherical carrier which has the pore structure almost identical to that of the major raw material such as alumina hydrate gel, silica-alumina hydrate gel, zirconia-alumina hydrate gel, titania-alumina hydrate gel, alumina hydrate paste, boria-alumina hydrate paste, and boria-silica alumina hydrate paste. The spherical catalyst carrier is composed of any of alumina, silica-alumina composition, zirconia-alumina composition, titania-alumina composition, boria-alumina composition, and boria-silica-alumina composition.
2. Description of the Related Art
A spherical carrier made of alumina is used as an adsorbent, catalyst, or catalyst carrier in the chemical industry and petrochemical industry. Also, a carrier of any of silica-alumina composition, zirconia-alumina composition, titania-alumina composition, boria-alumina composition, and boria-silica-alumina composition is used more extensively in the above-mentioned technical fields because of its larger specific surface area, better acid resistance and heat resistance, and higher solid acid concentrations as compared with the above-mentioned alumina carrier.
In general, a spherical carrier supporting a catalytic active metal is filled in a fixed-bed reactor or moving-bed reactor. Being spherical, it is uniformly filled in the reactor and easily removed from the reactor. Moreover, it does not cause channeling (a phenomenon that the reactant flows along a bypass during operation). It is also essential for reactions in a moving-bed reactor in which the reactant flows or moves downward by gravity.
Such a spherical carrier as mentioned above is produced by xe2x80x9cdropping in oilxe2x80x9d process or tumbling granulating process. According to the former process, an alumina spherical carrier is produced by gelling alumina sol in an alkaline atmosphere with hexamethylenetetramine or urea which decomposes at high temperatures to give off ammonia. The resulting spherical carrier has a smooth surface and is almost truly spherical. The gel made from alumina sol is homogeneous (from the surface to the inside) and has micropores with a diameter smaller than 50xc3x85.
The tumbling granulating process involves the steps of tumbling properly moistened alumina hydrate powder, thereby forming granules, and spraying water and supplying properly moistened alumina hydrate powder alternately, thereby thickening the layers and growing the granules. It yields a spherical carrier with a slightly rough surface and a broad distribution of sphericity. Granules formed by tumbling consist of layers and pores varying in size (ranging from macro to micro). Despite these disadvantages, it is widely used because of the low production cost.
A spherical carrier for the moving-bed reactor flows and/or moves during reaction and hence is required to have good wear resistance and a uniform, smooth surface. Therefore, it is produced mainly by xe2x80x9cdropping in oilxe2x80x9d process. Despite its smooth surface and homogeneity, a spherical carrier made by xe2x80x9cdropping in oilxe2x80x9d process contains micropores (smaller than 50xc3x85 in diameter) which account for a large portion. Such micropores are undesirable in some application areas, such as waste gas purification, in which physical poisoning and reduction in catalytic activity occur due to pore clogging.
In general, the performance of a catalyst is closely related with the pore characteristics of the carrier. Consequently, the carrier should have physical properties required of individual reaction conditions.
The present invention was completed to address the above-mentioned problems. It is an object of the present invention to provide a process for producing a spherical catalyst carrier of alumina, silica-alumina composition, zirconia-alumina composition, titania-alumina composition, boria-alumina composition, or boria-silica-alumina composition, from alumina hydrate gel, silica-alumina hydrate gel, zirconia-alumina hydrate gel, titania-alumina hydrate gel, alumina hydrate paste, boria-alumina hydrate paste, or boria-silica-alumina hydrate paste. The carrier has almost the same pore structure as that of the raw material, uniform sphericity, smooth surface, and good homogeneity. In addition, the carrier permits its micropore volume to be controlled as desired.
The gist of the present invention resides in a process for producing a spherical catalyst carrier which comprises adding a polysaccharide solution to any of alumina, silica-alumina, zirconia-alumina, or titania-alumina in the form of hydrate gel, mixing them to form a slurry with a controlled concentration, dropping the slurry into a solution containing multivalent metal ions, thereby forming spherical hydrogel, and performing the additional steps of aging, washing, drying, and calcining. According to the present invention, the polysaccharide solution to be added to the hydrate gel is a 1.0-3.0 wt % solution of low-methoxyl pectin or sodium alginate and the amount of the polysaccharide solution is 3-10 times the amount of the hydrate gel in terms of oxide (by weight), and the slurry contains 5-20 wt % of alumina, silica-alumina, zirconia-alumina, or titania-alumina in terms of oxide.
According to the present invention, the above-mentioned process is modified such that the polysaccharide solution is added to and mixed with an alumina hydrate paste which is formed from alumina hydrate gel and additional alumina hydrate powder. In this case, the amount of alumina hydrate powder (in terms of oxide) is 20-60 wt % of the alumina hydrate gel (in terms of oxide), the polysaccharide solution to be added to the alumina hydrate paste is 1.0-3.0 wt % solution of low-methoxyl pectin or sodium alginate, and the amount of the polysaccharide solution to be added to the alumina hydrate paste is 3-10 times the amount of the alumina hydrate gel in terms of oxide (by weight).
According to the present invention, the above-mentioned process is modified such that the polysaccharide solution is added to and mixed with a boria-alumina hydrate paste which is formed from the alumina hydrate gel and additional boron or a boria-silica-alumina hydrate paste which is formed from the silica-alumina hydrate gel and additional boron. In this case, the polysaccharide solution to be added to the boria-alumina hydrate paste or boria-silica-alumina hydrate paste is 1.0-3.0 wt % solution of low-methoxyl pectin or sodium alginate, and the amount of the polysaccharide solution to be added to the boria-alumina hydrate paste or boria-silica-alumina hydrate paste is 3-10 times the amount of the hydrate paste in terms of oxide (by weight).
According to the present invention, the slurry contains 5-20 wt % of boria-alumina or boria-silica-alumina in terms of oxide, and the solution of multivalent metal ions contains 0.5-3.0 wt % of at least one of calcium, aluminum, magnesium, barium, or strontium. After washing, the spherical alumina or silica-alumina hydrogel is hydrogel is dried at 60-120xc2x0 C. and calcined at 500-900xc2x0 C. Alternatively, after washing, the spherical zirconia-alumina hydrogel or titania-alumina hydrogel is dried at 60-120xc2x0 C. and calcined at 400-700xc2x0 C. Alternatively, after washing, the spherical boria-silica-alumina hydrogel is dried at 60-120xc2x0 C. and calcined at 500-1,400xc2x0 C.